Bugfix in bb_a21.c

Created by: gvarlet

When used with several M-Modules, each time a M_open()/M_close() is made on any device on one of the 3 slots of the A21B, the IRQ enable bit was cleared for all slots when A21_BrdInit()/A21_BrdExit() were called. Change MWRITE_D8 to MSETMASK_D8 to do not overwrite the IRQ Enable bit.